1. Field of the Invention
The present invention relates to a resonator-type elastic wave filter utilizing, for example, a boundary acoustic wave or a surface acoustic wave. More particularly, the present invention relates to an elastic wave filter utilizing an elastic wave in a mode for obtaining a desired passband and an elastic wave in a mode different from the mode for obtaining the desired passband.
2. Description of the Related Art
Various resonator-type surface acoustic wave filters have been disclosed in order to provide size reduction and loss reduction. For surface acoustic wave filters of this type, a significant attenuation amount near the passband and outside the passband is highly desired. In order to increase an attenuation amount outside the band, for example, in Japanese Unexamined Patent Application Publication No. 6-260876, a surface acoustic wave filter apparatus is disclosed in which a trap-type series resonator is connected to a surface acoustic wave filter.
FIG. 9 is a plan view schematically showing an electrode structure of a surface acoustic wave filter apparatus of this type.
In a surface acoustic wave filter apparatus 101 shown in FIG. 9, an electrode structure is arranged on a piezoelectric substrate. That is, a trap-type series resonator 104 is connected between a first-stage resonator-type surface acoustic wave filter 102 and a second-stage resonator-type surface acoustic wave filter 103.
Each of the surface acoustic wave filters 102 and 103 is a 3 IDT resonator-type surface acoustic wave filter which includes three IDTs 102a through 102c or 103a through 103c. Reflectors 102d and 102e are arranged on both sides of a region in which the IDTs 102a through 102c are provided along a surface-wave propagation direction. Similarly, reflectors 103d and 103e are arranged on both sides of a region in which the IDTs 103a through 103c are provided along a surface wave propagation direction.
On the other hand, the series resonator 104 is a one-port SAW resonator which includes an IDT 104a and reflectors 104b and 104c, which are arranged at both sides of the IDT 104a along propagation of a surface wave.
The first stage and the second stage surface acoustic wave filters 102 and 103 are cascade connected between an input terminal 105 and an output terminal 106. However, the trap-type series resonator 104 is connected in series between the surface acoustic wave filters 102 and 103. The trap-type series resonator 104 includes a trap which has a significant attenuation amount in an attenuation area near the passband of a two-stage cascade-connection surface acoustic wave filter apparatus which includes the surface acoustic wave filters 102 and 103. Thus, the attenuation amount outside the passband can be increased, and a steepness of the attenuation characteristics at an end portion of the passband can be increased.
In the surface acoustic wave filter apparatus 101 of the related art, the attenuation amount near the passband can be increased by connecting the trap-type series resonator 104. However, an electrode structure for providing the trap-type series resonator 104 is required, and thus, the chip size of the surface acoustic wave filter apparatus 101 must be increased. In addition, by connecting the series resonator 104, there is a problem that an insertion loss in the passband is increased.
On the other hand, recently, not only surface acoustic wave filters but also boundary acoustic wave filters utilizing a boundary acoustic wave have been known as elastic wave filters. Similarly, for the boundary acoustic wave filters, without increasing the size thereof, the attenuation amount outside the passband can increased.